Why is From in Sent Mail Showing as Yahoo! Mail Inc and not my username?

Can't tell you really how and why, but here's what you can do to change it back...

1: Click Options in the upper-right corner of your mail page, select "mail/more options".

2: From the list on the left, click "accounts".

3: Click "Yahoo! Mail" under accounts.

4: In the "your 'From' name" field, enter your name exactly as you'd like it to appear within your outgoing messages.

5: If you leave the Reply-To field blank, your Yahoo! Mail address will be automatically included as the return address in your outgoing messages. If you'd like a different return address to be included, enter it in the Reply-To field

6: Click the Save button to put your new settings into effect.

Note 1: you have to write something in the "name" field, because if you don't, whatever was written there before will be restored and your changes won't be saved. Please note that it isn't possible to 'hide" your email address when sending messages from a Yahoo! Mail account. When you send a message, your address will appear in the "from" field in the header of the message.

Note 2: "from" can't contain special characters like the "@" symbol. And the address in "reply to" can't exceed 40 characters.